Fiber laser welding of dissimilar materials between AISI316L austenitic stainless steel and EH36 ship steels were conducted. Then the effects of welding speed on microstructure and mechanical characterization of the welded joint were investigated. Optical microscopy, Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) were used to analyze the microstructure. Microhardness testing, transverse tensile strength, and impact tests at the temperature of −40 °C were performed to study the mechanical properties. The martensite phase formed due to the rapid cooling rate during laser welding and low Cr_eq/Ni_eq ratio. The coarse martensite grains in the center seam are transformed to finer martensite grains as the welding speed increases resulting in the higher cooling rate. The microhardness of joints was about 350 HV_0.3, which was twice that of the base metal because of the formation of the martensite phase. When the welding speed was 0.6 m/min, fewer defects were found, and tensile testing indicated overmatching of the weld metal relative to the base metal. In addition, the joints also exhibited better ductility and impact toughness.